1. Field of the Invention
The present invention relates generally to a roller band type (Rolamite) acceleration sensor, and in particular to a roller band acceleration sensor which has a failsafe feature.
2. Description of Related Art
Roller band type acceleration sensors (known as "Rolamite") have many uses. These sensors have a low coefficient of friction which makes them ideal for many applications because very little energy is required to operate the acceleration sensor.
Several patents disclose a basic roller band type structure. The first Rolamite patent was granted on Jun. 24, 1969 to Donald F. Wilkes as U.S. Pat. No. 3,452,175 and is assigned to the U.S. Department of Energy and is incorporated herein by reference. The Wilkes patent discloses the basic rolamite design which includes a two roller system and a tension band in between the two rollers. A switch is provided at one end of the rolamite housing. Various functional cut outs within the band are shown which effect the characteristics of the Rolamite.
U.S. Pat. No. 3,567,881 to Duimstra et al, which is incorporated herein by reference and assigned to the same assignee as this application, discloses a roller band inertial switch which has a housing, a pair of rollers, a tension band intertwined between the rollers, a reset end cap and a switch end. However, neither of these patents disclose a way of preventing the Rolamite from failing in a conducting position should an excessive acceleration occur.
Rolamite roller band acceleration sensors have been fabricated in a number of ways over the years. U.S. Pat. No. 3,688,063 to Bell discloses a crash sensing switch which is a variation of the original roller band type design. Bell discloses a switch which includes a resettable latch, a force biasing adjustment means and a separate switching mechanism which does not directly use the band to make an electrical contact as was done in the original rolamite patent to Wilkes. This switch does not provide a mechanism for preventing a sudden excessive acceleration from causing the switch to be jammed in a conducting or closed position.
U.S. Pat. No. 4,116,132 to Bell discloses an inertial sensor which includes a rolamite device. A cocked spring mechanism releases the sensor depending on the amount of acceleration. The sensor also includes a safety interlock mechanism. Energy to engage the sensor is stored mechanically in the cocked spring which must be overcome. If the band within the sensor breaks, the energy of the spring is released without enabling the sensor or actuating the switch. The rolamite device in this patent is not actually part of the electrical circuit and thus only indirectly prevents an electrical circuit from being closed should an excessive acceleration occur.
A Rolamite has a band which may break because of excessive acceleration. This breaking can prevent the sensor or switch from being activated or cause sticking in a closed (conductive) position. A number of devices which have frangible elements are known in other arts. U.S. Pat. No. 2,802,073 to Simon discloses an auto safety belt which has a conductive band which permits power to be transmitted through the band when the seat belt is fastened. The belt also includes a disconnect feature which interrupts power when a given tension is exceeded. Thus, the band is a part of an electrical circuit which is broken when excessive forces occur. This disconnect feature is resettable and may reclose once the excessive force is relieved. This makes the belt unpredictable. Moreover, this device does not act as an acceleration sensor prior to breakage.
U.S. Pat. No. 4,698,623 to Smith discloses an overload detecting apparatus which incorporates a frangible container of a visibly dense, conductive liquid whose presence is sensed electrically/optically when the container is broken. The container has a weakened area which fractures when a certain level of acceleration occurs. This apparatus does not act as an acceleration sensor prior to breakage and requires a liquid which could cause reliability and leakage problems as well as being expensive to contain.